The present invention concerns a process for the regeneration of chemical conversion catalysts used in a cyclic process for the production of hydrogen peroxide.
It is well known that the production of hydrogen peroxide using quinones is a cyclic process, whereby a quinone is dissolved in an organic solvent, hydrogenated to hydroquinone, and then oxidized by air or oxygen into quinone hydroperoxide. This, on contact with water, provides an aqueous solution of hydrogen peroxide and an organic solution containing regenerated quinone. The solution which goes through the hydrogenation-oxidation-extraction cycle is called the work solution. The quinones used are generally alkylated anthraquinones and particularly the 2-alkyl anthraquinones.
In the hydrogen peroxide production cycle, undesirable secondary reactions occur in addition to the principal reactions, and result in degradation of the quinone and the solvents used.
Because of (a) the high cost of substituted anthraquinones and (b) the generally prejudicial effect of the accumulation of products detrimental to the properties of the work solution, it is necessary to treat the work solution so as to prevent its deterioration, or at least to keep its degradation to a minimum. The secondary reactions are normally reversible; it is possible to reverse the reaction direction by modifying the conditions of the reaction medium. One widely used technique consists of treating the work solution at a relatively high temperature (80.degree. to 200.degree. C.) on a solid catalyst; this is preferably carried out on a branch of the main circuit, called the conversion circuit.
The conversion catalysts are generally neutral or slightly alkaline microporous solids; these are often aluminas of various structures, but the gels of alkaline aluminosilicates are also widely used and are becoming increasingly important.
When a catalytic mass is installed in a conversion circuit, the activity of the catalyst always increases during the first few hours after installation; this activity then decreases with prolonged use. After several days of use, the catalytic charge must be renewed. It is known that these microporous catalysts, which are adsorbent, have a purifying effect on the work solution, but have a strong tendency to clog up.
The renewal of these catalyst charges presents several problems to the manufacturer. These consist principally of the purchase of a new catalyst, which represents an important part of the operational cost, and the disposal of the used catalyst, which is impregnated with organic matter, particularly toxic phenols, which prevents it simply being dumped or discharged because of the risk of polluting water and the environment. Thus spent catalyst cannot be disposed of without preliminary detoxification treatment. The only simple and effective treatment hitherto has been combustion of the spent catalyst in air, resulting in an inert residue. However, combustion of the microporous catalysts used in these cycles for the production of hydrogen peroxide is not entirely satisfactory, for the major part of the organic material impregnated in the catalyst starts to distil when heated to between 280.degree. and 500.degree. C., causing an abundant yellow aerosol to form which is unpleasant and toxic.
Finally, if one considers that these catalysts contain 75 to 90% inorganic residue, it will be appreciated that they can not be considered as fuels with an acceptable calorific or economic value, and their disposal by specialist industrial waste disposal techniques has an unfavorable influence on the economics of producing hydrogen peroxide.
In 1962, in French Pat. No. 1,304,901 a process for the reactivation of conversion catalysts was proposed. According to this process, the catalyst, constituted by an aluminosilicate of sodium, is first of all washed with an appropriate solvent at 80.degree. C., then the solvent is removed by treating the catalyst with a current of steam at 130.degree. C.; the catalyst is then heated, preferably to between 400.degree. and 450.degree. C. in the presence of air for longer than an hour, more preferably for 8 to 12 hours. The conversion catalyst, regenerated under these conditions, shows interesting catalytic activity compared to virgin catalyst. However, catalyst treated by this process contains carbon residues and reduction products such as sulphides, which are undesirable when the catalyst is used in the production of hydrogen peroxide. For example, the sulphides tend to be carried over and inhibit the activity of the catalyst used in the hydrogenation phase of the process, this catalyst usually being prepared from Group VIII metals such as nickel and palladium.